The present invention generally relates to semiconductors and more particularly to a driver circuit of a tunable laser diode.
The inventors of the present application have proposed previously in the U.S. patent application Ser. No. 552,116, now abandoned for Ser. No. 789,427, now U.S. Pat. No. 5,170,402 and in the corresponding European patent application No. 90307626.3, a tunable laser diode that has a DFB corrugation and includes a plurality of electrodes separated from each other electrically and provided in tandem along the resonant structure. The foregoing references are incorporated herein by reference.
In this laser diode, a non-uniform distribution of photons, and hence of carriers, is induced in the resonant structure such that the carrier density becomes a minimum in correspondence to a central part of the structure. A first electrode is provided in correspondence to such minimum of the carrier density. It should be noted that the minimum of the carrier density corresponds to the maximum of the photon density. Further, in correspondence to the maximum of the carrier density occurring at the opposite ends of the laser diode, second and third electrodes are provided. The first electrode injects a first drive current while the second and third electrodes inject a second drive current, wherein the first drive current is set substantially smaller than said second drive current thereby to cause the desired non-uniform distribution of the carriers. Further, a modulation signal is superposed on the first drive current. By supplying the modulation signal to the region where the carriers are depleted, a large shift of the oscillation wavelength is achieved in response to the modulation signal.
FIG. 1 shows the foregoing tunable laser diode wherein the laser diode includes an n-type InP substrate 1 on which a waveguide layer 2 of n-type GaInAsP is provided. On the waveguide layer 2, there is provided an active layer 3 of undoped GaInAsP and a clad layer 4 of p-type InP is provided on the active layer 3. Thus, the diode has a double-hetero p-n junction and a corrugated grating is formed at an interface between the substrate 1 and the waveguide layer 2. On the clad layer 4, an electrode 17 is provided in correspondence to a central longitudinal part, or portion of the structure as the first electrode, and respective electrodes 16 and 18 are provided at the opposite sides thereof, in the longitudinal direction, with a separation therebetween.
In order to induce the non-uniform distribution of the carriers and to cause the non-uniform distribution of the photons in the laser diode, a drive current Is is supplied to the electrodes 16 and 18 together with a drive current Ic that is supplied simultaneously to the electrode 17. By setting the magnitude of the currents Is and Ic suitably, the desired distribution of the carriers is obtained.
FIG. 2 shows another example of the conventional tunable laser diode, wherein a number of electrodes 10-15 are provided on the upper major surface of the clad layer 4.
In this laser diode, too, a non-uniform distribution of the carriers is induced in the laser diode along the longitudinal direction, wherein the electrodes 12 and 13 at the center are provided in correspondence to the minimum of the carrier density and the current Ic is supplied thereto, similarly to the device of FIG. L. On the other hand, other electrodes 10-11 and 14-15 are supplied with the current Is.
FIG. 2 also shows the cross section of the laser diode. As shown therein, the essential part of the laser diode, such as the upper part of the substrate 1, on which the corrugation grating is formed, the active layer 3, and the clad layer 4, forms a stripe region or mesa structure 8 extending in the longitudinal direction, and both of the opposite sides of the mesa structure 8 are filled by a p-type buried layers 5 and 6 of InP. Further, a common electrode 9 is provided on the bottom surface of the substrate 1.
FIG. 3 shows another example of the conventional laser diode, wherein the laser diode is formed from three segmented parts 21, 22 and 23 that are arranged to share a common optical axis 24. The parts 21-23 carry thereon electrodes 21a-23a. In this structure, too, the non-uniform distribution of the carriers is induced in the array of the parts 21-23, by supplying the drive current Is to the electrodes 21a and 23a while supplying the drive current Ic to the electrode 21b.
It should be noted that the foregoing laser diodes are characterized by a frequency modulation operation wherein the oscillation frequency changes in response to the current injected to the electrodes, e.g., 16-18 in FIG. 1. In the frequency modulation operation of the laser diode, there exist three independent parameters, i.e. the oscillation frequency, the output power and the modulation efficiency, wherein the modulation efficiency eff is defined as a frequency shift .DELTA.f divided by a current variation .DELTA.IC that has caused the frequency shift .DELTA.f (eff=.DELTA.f/.DELTA.Ic).
FIG. 4 shows the relationship between the oscillation wavelength .lambda. and the injection current Is for various values of the injection current Ic. As shown in FIG. 4, the oscillation wavelength .lambda. changes in response to both the current Ic and the current Is, wherein the line designated as M indicates the operational point on which the modulation efficiency eff is held constant. Along the line M, one can control the oscillation wavelength of the laser diode while maintaining the modulation efficiency eff constant. It should be noted that, when the current Ic is changed while holding the current Is constant, both the output power and the modulation efficiency change in response to the current Ic. This reflects the situation that there are three independent parameters (oscillation wavelength, output power and the modulation efficiency) of laser operation while there are only two variables, Ic and Is, that are controlled by the external drive circuit. The remaining independent parameter of the laser oscillation is the temperature of operation. In the foregoing description, it is assumed that the temperature of the laser is held constant during the operation.
FIG. 5 shows the relationship between the output power and the current Is for various values of the current Ic. As will be noted, the output power changes when the current Ic is changed while holding the current Is constant. In FIG. 5, the line designated by N indicates the line along which one can change the output power while holding the modulation efficiency constant. Again, it is assumed that the temperature of the laser is held constant.
FIG. 6 shows the relationship between the modulation efficiency eff and the current Is for various values of the current Ic. As can be seen in FIG. 6, the value of the modulation efficiency changes generally with the current Ic and the current Is. For example, the modulation efficiency changes between .DELTA.f1 and .DELTA.f2 when the current Ic is changed from Ic4 to Ic5 while holding the value of the current Is at Is1. On the other hand, the modulation efficiency eff is held constant on a line designated P when the current Ic and the current Is are changed simultaneously with a predetermined relationship.